How about Melody?
I also kinda like older names likes Meredith or Matilda, but a more fashionable MacKenzie, might sound better with Madison. Or maybe McKenna...?
The cute nickname Maggie, made me want to say Margaret, though i wonder if your daughter may already get 'Maddie' and thats pretty similar. And Madeline is another classic name i like, but again, so similar to Madison.